Unwanted electromagnetic radiation has been a problem encountered by various information apparatuses in recent years. Since the primary electromagnetic radiation has a frequency spectrum corresponding to that of harmonics generated by a clock on a printed wiring board, it has been considered that electromagnetic radiation is caused mainly by a signal line for transmitting a clock signal or a digital signal synchronous with the clock signal. Therefore, various electromagnetic radiation preventing measures have been taken to suppress electromagnetic radiation from signal lines formed on printed wiring boards and wire harnesses connected to those signal lines.
More specifically, those electromagnetic radiation preventing measures include (1) subjecting signals including clock signals and digital signals to low-pass filtering process to pass only signals of frequencies in a necessary band, (2) combining a damping resistor to an output signal line to increase the rise time and the fall time of signals and (3) forming a guard pattern of a ground potential near a signal line to form a small feedback current loop.
However, as is generally known, a frequency distribution of electromagnetic waves actually detected on a printed wiring board is different from a frequency distribution of electromagnetic waves estimated from a distribution of currents on signal lines, and the frequency distribution curve has a sharp peak for a specific frequency independently of the properties of the signal line. Therefore, it is expected that a main cause of electromagnetic radiation from a printed wiring board is not the signal lines but the electrical power system of the printed wiring board; that is, the main cause of electromagnetic radiation is the electrical resonance between the power source layer and the ground layer of the printed wiring board.
It is apparent that the foregoing conventional radiation preventing measures taken to suppress electromagnetic radiation from the signal lines of a printed wiring board and wire harnesses connected to those signal lines are ineffective in preventing electromagnetic radiation caused by the power source layer and the ground layer of the printed wiring board.
A method of suppressing electromagnetic radiation caused by the electrical power system of a printed wiring board, proposed in "Low-EMI Multilayer Printed-circuit Board", Kairo Jisso Gakkai Taikai Ronbun-shi, Vol. 10, p.175 uses a printed wiring board of a structure shown in FIG. 19.
In a printed wiring board for carrying out the method of suppressing electromagnetic radiation previously proposed in "Low-EMI Multilayer Printed-circuit Board", two ground layers 32A and 32B are formed on the opposite sides of a power source layer 31, respectively, resistors 33 are connected to end portions of the ground layers 32A and 32B so as to connect the ground layers 32A and 32B. Dielectric layers 34A and 34B of different dielectric constants are formed between the power source layer 31 and the ground layer 32A and between the power source layer 31 and the ground layer 32B, respectively, to suppress electrical resonance between the power source layer 31 and the ground layers 32A and 32B.
The structure of the printed wiring board shown in FIG. 19, which is fabricated by forming the two ground layers 32A and 32B on the opposite sides of the power source layer 31, respectively, connecting the resistors 33 to end portions of the ground layers 32A and 32B so as to connect the ground layers 32A and 32B, forming the dielectric layers 34A and 34B of different dielectric constants between the power source layer 31 and the ground layer 32A and between the power source layer 31 and the ground layer 32B, respectively, is greatly different from that of general printed wiring boards. Furthermore, different matching conditions for suppressing electromagnetic radiation must be determined for different printed wiring boards of the foregoing structure differing from each other in size and shape. Therefore, it is very difficult to apply the structure shown in FIG. 19 to practical printed wiring boards and, even if the structure is applied to printed wiring boards, the printed wiring boards of such a structure will require a very high manufacturing cost.
Accordingly, it is an object of the present invention to provide a printed wiring board device for use in constructing an electronic equipment, such as information equipment capable of effectively suppressing electromagnetic radiation caused by the power source layer and the ground layer of a printed wiring board included therein and difficult to suppress by the conventional electromagnetic radiation suppressing measures taken to suppress electromagnetic radiation from signal lines formed on printed wiring boards and wire harnesses connected to those signal lines, of being applied to general purposes and of being fabricated at a low cost, having a structure similar to that of general printed wiring boards, and not requiring the setting of any special condition special for the size and the shape thereof.